Ch 17 Transcription Translation BIO1300 PDF

Ch. 17 Gene Expression = Transcription + Translation Flow of Genetic Info from Gene to Protein Information content of DNA is in the form of specific sequences of nucleotides – DNA inherited by an organism leads to specific traits by dictating the synthesis of pro...

Ch. 17 Gene Expression = Transcription + Translation Flow of Genetic Info from Gene to Protein Information content of DNA is in the form of specific sequences of nucleotides – DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins – Proteins links genotype and phenotype What is a Gene? A region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule – Gene expression, the process by which DNA directs protein synthesis Watch video by yourgenome 2015 (2:41) “From DNA to Protein” https://www.youtube.com/watch?v=gG7uCskUOrA Transcription Translation © 2011 Pearson Education, Inc.  Concept 17.1: Genes specify proteins via transcription & translation RNA = bridge between genes & proteins for which they code Transcription = synthesis of RNA using information in DNA – Transcription produces messenger RNA (mRNA) Translation is the synthesis of a polypeptide, using information in the mRNA – Ribosomes are the sites of translation In prokaryotes, translation of mRNA begins before transcription has finished In eukaryotic cells, nuclear envelope separates transcription from translation – Eukaryotic primary RNA transcripts are modified through RNA processing to yield the finished mRNA that then leaves nucleus Google HHMI Biointeractive, @ Site hhmi.org, use search box for these 2 videos: DNA Transcription Video (run time 1:55) Translation Video (3:04 run time), view the advanced version © 2011 Pearson Education, Inc.  Central Dogma of Molecular Nuclear envelope Biology: cells are governed by cellular chain of command: DNA DNA ® RNA ® Protein TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Ribosome TRANSLATION Polypeptide Polypeptide (a) Bacterial cell Fig. 17.4 (b) Eukaryotic cell The 1 Gene 1 Polypeptide Hypothesis = Many proteins are composed of several polypeptides, each having its own gene Types of RNA Messenger RNA (mRNA) Produced in the nucleus from DNA template during TRANSCRIPTION Carries genetic message out of nucleus to ribosomes Transfer RNA (tRNA) Found in cytoplasm, transfers amino acids to ribosomes during TRANSLATION Each type of tRNA carries only one type of amino acid Ribosomal RNA (rRNA) Produced in the nucleolus inside the nucleus, then goes to cytoplasm Joins with proteins to form ribosomes Ribosomes may be free in the cytoplasm, or in polyribosomes (clusters) or attached to ER (endoplasmic reticulum) Ribozyme (a catalytic RNA) Spliceosome cuts out introns from mRNA 11-4 Copyright © McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or distribution without the prior written consent of McGraw-Hill Education. The Genetic Code & Codons: Triplets of Nucleotides How are the instructions for assembling amino acids into proteins encoded into DNA? – There are 20 amino acids, but only 4 nucleotide bases How many nucleotides correspond to an amino acid? Genetic Code: flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, 3-nucleotide “words” Amino acid “Words” of a gene are transcribed into complementary nonoverlapping 3-nucleotide “words” of mRNA These “words” are then translated into a chain of amino acids, forming a polypeptide mRNA © 2011 Pearson Education, Inc.  Chromosome Figure 17.5 Triplet Code mRNA is complementary to template DNA strand (U replaces T in RNA) AUG is the mRNA start codon!  During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript – Template strand is always the same strand for a given gene During translation, the mRNA base triplets, called codons, are read in 5¢ to 3¢ direction (see CODON Table next slide Fig. 17.6) – Codons along an mRNA molecule are read by translation machinery in the 5¢ to 3¢ direction – Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide © 2011 Pearson Education, Inc.  Second mRNA base U C A G Fig. 17.6 UUU UCU UAU UGU U Phe Tyr Cys CODON UUC UCC UAC UGC C U Ser Table for UUA UCA UAA Stop UGA Stop A mRNA Leu Third mRNA base (3¢ end of codon) First mRNA base (5¢ end of codon) UUG UCG UAG Stop UGG Trp G All 64 codons were CUU CCU CAU CGU U His deciphered by CUC CCC CAC CGC C C Leu Pro Arg the mid 1960s CUA CCA CAA CGA A Gln CUG CCG CAG CGG G 64 triplets, 61 code for AUU ACU AAU AGU U amino acids Asn Ser AUC Ile ACC AAC AGC C A Thr 3 triplets are AUA ACA AAA AGA A Lys Arg “stop” signals AUG Met or ACG AAG AGG G start to end translation GUU GCU GAU GGU U Asp GUC GCC GAC GGC C G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G Cracking codons Amino acid THE GENETIC CODE Genetic code is redundant (more than 1 codon may specify a particular amino acid) but not ambiguous as no codon specifies more than 1 amino acid – Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced Genetic code is nearly universal, shared by the simplest bacteria to the most complex animals – Genes can be transcribed and translated after being transplanted from one species to another © 2011 Pearson Education, Inc. Central Dogma of Molecular Biology: DNA ® RNA ® Protein Nuclear envelope What is a Gene? A region of DNA that can be expressed to produce a final functional product, either a DNA TRANSCRIPTION polypeptide or an RNA molecule Pre-mRNA RNA PROCESSING mRNA DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Ribosome TRANSLATION Polypeptide Polypeptide (a) Bacterial cell Fig. 17.4 (b) Eukaryotic cell The 1 Gene 1 Polypeptide Hypothesis = Many proteins are composed of several polypeptides, each having its own gene Concept 17.2: Transcription is the Fig. 17.8 DNA-directed synthesis of RNA Transcription The 3 stages of transcription – Initiation – Elongation – Termination RNA transcript is the mRNA mRNA is 5’ to 3’ as complementary to the DNA template strand that is 3’ to 5’ © 2011 Pearson Education, Inc. Details of Transcription RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides – The RNA is complementary to the DNA template strand The DNA sequence where RNA polymerase attaches is called the promoter – As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time The stretch of DNA that is transcribed is called a transcription unit © 2011 Pearson Education, Inc. Elongation of the RNA Strand Transcription rate is 40 nucleotides per second in eukaryotes Nucleotides are added to the 3¢ end of the growing RNA transcript RNA synthesis follows the same base-pairing rules as DNA, except that uracil (U) substitutes for thymine (T) Fig. 17.10 Transcription elongation © 2011 Pearson Education, Inc. Concept 17.3: Eukaryotic cells modify RNA after transcription Enzymes in the eukaryotic nucleus modify pre-mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm – During RNA processing, both ends of the primary transcript are usually altered – Usually some interior parts of the molecule are cut out, and the other parts spliced together © 2011 Pearson Education, Inc.  RNA Processing: Alteration of pre-mRNA Each end of a pre-mRNA molecule is modified – 5¢ end gets modified nucleotide phosphorylated 5¢ cap – 3¢ end gets a poly-A tail These modifications function to: – Facilitate the export of mRNA to the cytoplasm – Protect mRNA from hydrolytic enzymes – Help ribosomes attach to the 5¢ end Fig. 17.11 RNA Processing © 2011 Pearson Education, Inc. RNA Processing: Introns vs. Exons Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions These noncoding regions are called intervening sequences, or introns The other regions are exons (protein coding segments) because they are eventually expressed, usually translated into amino acid sequences RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence Fig. 17.12 RNA Processing  RNA splicing is carried out by spliceosomes – Spliceosomes are a type of Ribozymes, which are catalytic RNA molecules that function as enzymes and can splice RNA – The discovery of ribozymes rendered obsolete the belief that all biological catalysts were enzyme proteins Fig. 17.13 Spliceosome splicing a pre mRNA  Source: Sinaur Assc. 2006 The Cell, 4th ed. Functional & Evolutionary Importance of Introns This is the gene’s transcription unit RNA Processing (5’cap, 3’polyA tail + RNA splicing + likely Final mature Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing à Alternative RNA splicing Consequently, the # of different proteins an organism can produce is much greater than its # of genes. Ex. Humans have < 30,000 genes, but before this was determined, we thought there would be over 100,000 based on protein diversity. Over 90% of our genes are alternatively spliced! Structure & Function of Transfer RNA (tRNA) Molecules of tRNA are not identical Each carries a specific amino acid on one end Each has an anticodon on the other end tRNA anticodon base pairs with a complementary codon on mRNA tRNA molecule consists of a single RNA strand that is 80 nucleotides long Because of hydrogen bonds, tRNA actually twists & folds into a 3D molecule that is roughly L-shaped Fig. 17.16 tRNA © 2011 Pearson Education, Inc.  Figure 17.5 Triplet Code AUG is the mRNA start codon! Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) Enzyme aminoacyl-tRNA synthetase attaches the correct amino acid to the tRNA, thus creating a “charged” tRNA tRNAs transfer amino acids to the growing polypeptide at a ribosome In protein synthesis, Ribosomes facilitate correct match between tRNA anticodons & mRNA codons © 2011 Pearson Education, Inc. Fig. 17.18  Second mRNA base U C A G Fig. 17.6 UUU UCU UAU UGU U Phe Tyr Cys CODON UUC UCC UAC UGC C U Ser Table for UUA UCA UAA Stop UGA Stop A mRNA Leu Third mRNA base (3¢ end of codon) First mRNA base (5¢ end of codon) UUG UCG UAG Stop UGG Trp G All 64 codons were CUU CCU CAU CGU U His deciphered by CUC CCC CAC CGC C C Leu Pro Arg the mid 1960s CUA CCA CAA CGA A Gln CUG CCG CAG CGG G 64 triplets, 61 code for AUU ACU AAU AGU U amino acids Asn Ser AUC Ile ACC AAC AGC C A Thr 3 triplets are AUA ACA AAA AGA A Lys Arg “stop” signals AUG Met or ACG AAG AGG G start to end translation GUU GCU GAU GGU U Asp GUC GCC GAC GGC C G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G Initiation stage of TRANSLATION brings together mRNA, a tRNA with the 1st amino acid & 2 ribosomal subunits 2 ribosomal subunits (large & small) are made of proteins & ribosomal RNA (rRNA) First, a small ribosomal subunit binds with mRNA and a special initiator tRNA Then, small subunit moves along the mRNA until it reaches the start codon (AUG) Fig. 17.18 © 2011 Pearson Education, Inc.  Elongation of the Polypeptide Chain During the elongation stage, amino acids are added one by one to the preceding amino acid at C-terminus of growing chain – Addition has 3 steps: codon recognition, peptide bond formation, translocation – Translation proceeds along the mRNA in a 5′ to 3′ direction Fig. 17.20 Translation Elongation © 2011 Pearson Education, Inc. Termination of Translation Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome – The A site accepts a protein called a release factor – Release factor causes the addition of a water molecule instead of an amino acid – This reaction releases the polypeptide, and the translation assembly then comes apart Fig. 17.21 © 2011 Pearson Education, Inc. Polyribosomes A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome) – Polyribosomes enable a cell to make many copies of a polypeptide very quickly Fig. 17.23 © 2011 Pearson Education, Inc. Protein Folding and Post-Translational Modifications During and after protein synthesis, a polypeptide chain spontaneously coils and folds into its 3D shape Proteins may also require post-translational modifications before doing their job – Some polypeptides are activated by enzymes that cleave them – Other polypeptides come together to form the subunits of a protein – Other polypeptides have entities such as carbohydrates added to them © 2011 Pearson Education, Inc.  DNA TRANSCRIPTION 3¢ -A ly Po RNA 5¢ RNA polymerase transcript Exon RNA RNA transcript PROCESSING (pre-mRNA) Intron Aminoacyl- Fig 17.25 Pol y-A tRNA synthetase NUCLEUS Summary of Amino acid Gene AMINO ACID CYTOPLASM tRNA Expression: ACTIVATION Transcription mRNA Growing polypeptide +Translation Ca 5¢ p 3¢ in a A -A Aminoacyl ly P Po (charged) Eukaryotic E Ribosomal subunits tRNA Cell ap 5¢ C TRANSLATION E A Anticodon Codon Ribosome Concept 17.5: Mutations of one or a few nucleotides can affect protein structure and function Mutations are changes in the genetic material of a cell or virus – Point mutations are chemical changes in just 1 base pair of a gene Fig. 17.26 Sickle cell disease is an example of a MISSENSE mutation Missense mutations still code for an amino acid, but not the correct amino acid One nucleotide gone wrong in this sensitive location means a change in the amino acid © 2011 Pearson Education, Inc.  Second mRNA base U C A G Fig. 17.6 UUU UCU UAU UGU U Phe Tyr Cys CODON UUC UCC UAC UGC C U Ser Table for UUA UCA UAA Stop UGA Stop A mRNA Leu Third mRNA base (3¢ end of codon) First mRNA base (5¢ end of codon) UUG UCG UAG Stop UGG Trp G All 64 codons were CUU CCU CAU CGU U His deciphered by CUC CCC CAC CGC C C Leu Pro Arg the mid 1960s CUA CCA CAA CGA A Gln CUG CCG CAG CGG G 64 triplets, 61 code for AUU ACU AAU AGU U amino acids Asn Ser AUC Ile ACC AAC AGC C A Thr 3 triplets are AUA ACA AAA AGA A Lys Arg “stop” signals AUG Met or ACG AAG AGG G start to end translation GUU GCU GAU GGU U Asp GUC GCC GAC GGC C G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G Fig. 17.27 Nucleotide-pair substitution replaces 1 nucleotide and its partner with another pair of nucleotides – Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code Insertions & deletions are additions or losses of nucleotide pairs in a gene – They can have a disastrous effect on the resulting protein